Oxygen and CL
What is the permeability of AV Oasys/VITA?
103
What is the permeability of infuse?
107
What is the permeability of air optix aqua?
110
What is the permeability of ultra?
114
What is the permeability of daily total 1?
140
For Benjamin lenses, what is considered a medium Dk?
15-30
For Bennet lenses, what is considered a low Dk?
25-50
For Benjamin lenses, what is considered a high Dk?
31-60
For Bennet lenses, what is considered a medium DK?
51-99
For Benjamin lenses, what is considered a low Dk?
<15
For Bennet lenses, what is considered a high Dk?
>100
What is the permeability of XO?
>100
What is the permeability of optimum extra?
>100
What is the permeability of XO2?
>141
What is the permeability of menicon z?
>163
What is the permeability of focus dailies aquacomfort?
>26
What is the permeability of EO?
>58
What does D stand for?
>Diffusion coefficient, velocity >a flow rate
What is the polarographic method?
>Done in a chamber filled with air (O2 tension) >CL is the barrier between air and O2 free chamber >cathode measures the O2 tension on the other side
What does the AQUEOUS supply oxygen to?
>ENDOTHELIUM
What is the relationship between the water content and permeability in SILICONE HYDROGEL lenses?
>INVERSE RELATIONSHIP >increase water = decrease DK
What is the relationship between the water content and permeability in HYDROGEL lenses?
>LINEAR RELATIONSHIP Increase water = increase DK
What are the methods to assess oxygen supply?
>Physiological content >Dk/L >EOP - equivalent oxygen percentage
What is does K stand for?
>Solubility
What does the movement of the lens allow?
>allows O2 to enter >gets rid of metabolic wastes - such as CO2, debris, metabolites, and sloughed cells)
What does O2 tension depend on in the TEARS?
>atmosphere
What supplies oxygen to the cornea while awake (eyes open)?
>atmosphere - 21% O2 or 155 mmHg >aqueous -7% O2 or 55 mmHg
What is seen acutely in deprivation of oxygen to the cornea?
>corneal edema - epithelial Microcysts: REVERSE ILLUMINATION -Stromal edema -Endothelial folds
What is seen long-term in deprivation of oxygen to the cornea?
>corneal neovascularization - look at the superior cornea
What do the tears supply oxygen to?
>epithelium >stroma
What are the advantages of the coulometric method?
>fewer artifacts - eliminates most edge effect >measures environment vs environment s
What does O2 passing through the lens material depend on?
>function of lens O2 permeability (Dk) >function of the lens thickness (Dk/L)
What is the equivalent oxygen percentage (EOP)?
>in Vivo technique >measures oxygen uptake rates for known concentration of oxygen-goggles then measures the oxygen uptake rate for the test condition
What is Dk affected by?
>initial gas press ion (PPO2) >ambient temperature >surrounding atmospheric pressure (altitude)
For oxygen to be able to able to be dissolved in tears and mechanically moving under the lens with lid and eye movement what is needed?
>lens movement is needed - this hardly occurs in soft CL because they drape over the eye
What are the permeability discrepancies?
>liquid boundary effect - cigarette paper or water influence >thickness effect (Dk/l) - transmissibility >edge effects - happens because lens is bigger than cathode
A high altitudes what may occur to a CL wearer?
>may be more prone to suffer from short-term swelling
What is oxygen transmissibility?
>measure of the amount of oxygen through the lens >represented by Dk/l
What does this NOT factor into the calculation?
>not factor in blinking or pumping
What supplies the cornea while asleep (eyes closed)?
>palpebral conjunctiva - 7.5% or 55-60 mmHg
Which method overestimates O2 permeability?
>polarographic method
What do the limbal vessels do?
>provide a small amount of oxygen to the cornea when the eyes are closed
How can the assessment of oxygen supply be made physiologically?
>reduction in mitosis - implications for healing >decrease in corneal sensitivity
What is oxygen permeability?
>represented by Dk - not involve thickness - set at a specific thickness of 0.1 mm >O2 passing from one side of a material to other
What are some sources of oxygen to the cornea?
>tears (O2 dissolved) >aqueous >limbal vessels
What area of the cornea is MOST oxygen deprived?
>the area 1 mm below the superior limbus
What does the cornea maintain a majority of its oxygen from?
>the atmosphere
In EOP, what is the known oxygen concentration?
>the calibration curve then the test condition
What does the O2 tension depend on in the AQUEOUS?
>the differences between EPITHELIUM and ENDOTHELIUM
What are the problems of polarographic method?
>thin film of water with SCL when pushed on cathode >for RGP use teflon membrane or water-soaked cigarette power >overestimates?
What are the 2 routes of atmospheric O2 to the cornea while wearing a CL?
>through the ACTIVE tear pump >through the lens material
What is the coulometric method?
>winterton method
Can any lens have a Dk/L of greater than the Dk value?
>yes, if the center thickness is less than 1
What methods are used to measure permeability?
Both are in vitro -polarographic method -coulometric method
In what types of temperatures do CL have better decay?
Cold temperatures
As you increases the size of the CL, what happens to the tear exchange?
It decreases
In daily wear, to avoid edema or 0% sweeling during the day while the lens is being worn, what should the EOP be and Dk/T be normally? What about in a conservative way?
Normally: EOP = 10% and Dk/t = 24 (Holden-Mertz Criteria) Conservative number: barrow number Dk/t =35
In EW, to allow for overnight swelling of >4% but return to normal thickness (or 0% swelling during the day), what should the EOP be and Dk/T be normally? What about in a conservative way?
Normally: EOP = 18% and Dk/t = 87 Conservative Dk/t = 125
In which type of CL is the active tear pump MORE ACTIVE?
RGP - because the lens is moving more and is NON-DRAPING RGP: 10 to 20% SCL: 1 to 9 %
What is considered normal in relation to overnight swelling?
Up to 4%
What happens to the oxygen supply when you increase the thickness?
decreases oxygen supply
